Cenozoic deep crust in the Pamir - Earth Science

Transcription

Earth and Planetary Science Letters 312 (2011) 411-421
Contents lists available at SciVerse Science Direct
Earth and Planetary Science Letters
_LSEVIER
journal homepage: www.elsevier.comllocate/epsl
Cenozoic deep crust in the Pamir
Jennifer Schmidt a, Bradley R. Hacker a,*, Lothar Ratschbacher b, Konstanze Sttibner b, Michael Stearns c,
Andrew Kylander-Clark c, John M. Cottle c, A. Alexander G. Webb ct, George Gehrels e, Vladislav Minaev f
Earth Science, University of California, Santa Barbara, 93106, USA
Geologie, TU Bergakademie Freiberg, 0-09599 Freiberg, Germany
C Earth Science, University of California, Santa Barbara, CA, USA
d Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803, USA
e Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA
f Geology, Tajik Academy of Science, Dushanbe, Tajikistan
a
b
ARTICLE
INFO
Article history:
Received 31 August 2011
Received in revised form 20 October 2011
Accepted 21 October 2011
Available online 22 November 2011
Editor: T.M. Harrison
Keywords:
Pamir
metamorphic petrology
geochronology
exhumation
crustal recycling
ABSTRACT
Multiple high-grade crystalline domes across the Pamir contain Barrovian facies-series metapelites with peak
metamorphic assemblages of garnet + kyanite ± staurolite + biotite + oligoclase ± K-white mica. Thermobarometry yields pressures of 6.5-8.2 kbar and temperatures of 600-650 °C for the Kurgovat dome in the
northwestern Pamir, 9.4 kbar and 588°C for the west-central Yazgulom dome, 9.1-11.7 kbar and
700-800 °C for the east-central Muskol dome, and 6.5-14.6 kbar and 700-800°C for the giant Shakhdara
dome in the southwestern Pamir. These new data indicate exhumation of the Pamir crystalline domes
from crustal depths of -30-40 km. New titanite, monazite and zircon geochronology, in conjunction with
published ages, illustrate that this metamorphism is Oligocene-Miocene in all but the Kurgovat dome
(where it is Triassic). If the Pamir had a pre-collisional crustal thickness less than 30 km and if the IndiaAsia convergence within the Pamir is less than 600 km, the current 70 km-thick crust could have been created by plane strain with no net gain or loss of material. Alternatively, if the pre-collisional crustal thickness
was greater than 30 km or India-Asia convergence within the Pamir is more than 600 km, significant loss
of continental crust must have occurred by subhorizontal extrusion, erosion, or recycling into the mantle.
Crustal recycling is the most likely, based on deep seismicity and Miocene deep crustal xenoliths.
© 2011 Elsevier B.Y. All rights reserved.
1. Introduction
Much of our understanding of active continent-collision zones has
been derived from study of the Tibet Plateau, especially its southern
margin, the Himalaya. Tibet, however, has undergone minimal Cenozoic erosion, such that the processes active in the deep levels of the
collision zone remain largely a matter of conjecture. [n contrast, the
Pamir, farther west in the same collision zone (Fig. 1), are deeply dissected and expose large domains of high-grade crystalline rock. The
Pamir are virtually unknown compared to Tibet, but the large exposures of crystalline rock likely have much to tell us about processes
in the deep crust of continent collision zones.
Tibet and the Pamir both have 70 km-thick crust (Mechie et aI.,
2011, in review; Schurr et aI., 2009) and both absorbed
-2100-1800 km of Cenozoic India-Asia convergence (Johnson, 2002;
I.e Pichon et aI., 1992). The amounts of internal shortening are radically
different however. Cenozoic shortening within Tibet is -300-500 km
*
Corresponding author.
E-mail address: [email protected] (B.R. Hacker).
0012-821X/$ - see front matter © 2011 Elsevier B.V. All rights reserved.
doi: 10.1 016/j.eps1.2011.1 0.034
(DeCelles et aI., 2002), or -19-28% shortening for the 1300 km N-S
'width' ofTibet (Fig. 2A). Cenozoic convergence within the Pamir is difficult to quantify because of the dearth of investigations; however, if the
postulated -300 km southward underthrusting of the Tajik Depression
(Burtman and Molnar, 1993), - 700 km northward subduction oflndian
lithosphere (Negredo et aI., 2007), and <200 km shortening inferred for
the Pakistan Himalaya (DiPietro and Pogue, 2004) are correct, the Pamir
have absorbed -600-900 km of Cenozoic convergence (1800-2100
-300-700-200=600-900). This is -55-64% shortening, two to
three times that ofTibet (Fig. 2B-D).
Shortening in the Pamir might have produced enough crustal
thickening (Fig. 2C and D) to drive large-scale lateral extrusion (e.g.,
Ratschbacher et aI., 1991), exhumation by erosion/tectonic denudation, or recycling into the mantle (Negredo et aI., 2007). If so, the
high-grade rocks of the Pamir might be Cenozoic and might have
been exhumed from quite deep crustal levels. Alternatively, the preCenozoic crustal thickness of the Pamir might have been just
25-30 km (Burtman and Molnar, 1993), in which case the presentday 70 km-thick crust of the Pamir could have been built by homogeneous plane-strain vertical thickening with 55-64% shortening
(Fig. 2B). In this scenario, the high-grade rocks of the Pamir might
be dominantly pre-Cenozoic and only weakly exhumed.
412
J. Schmidt et al. / Earth and Planetary Science Letters 312 (2011) 411–421
Tajik Depression
Tarim Basin
71°E
38.5°N
76°E
Kurgovat
dome
Yazgulom
dome
4715A1 & B1
4716E1
4717A1-A3
4718A1 & B3
4727J1
6906A1
6818E2
n
96AK2e
9919A4b
9919G
dome
e
38°N
l P a m i r
a
Muskol
Sarez
t r
dome
Kongur Shan
dome
C
n
h e r
t
o u
4726H1
4727E1
6831A1 & A3
6831C1-C3
SShakhdara
6827C2
9910A1
6824G1
dome
37.5°N
6822D1
6821C1
6821M1
6821P2
6823A3
6823C1
N
100 km
Muztagh Ata
dome
Shatput
dome
Pa m
i r
Cenozoic crystalline rocks
Tertiary metavolcanic rocks
Tertiary granitoids
74°E
Cretaceous granitoids
Permian-Jurassic granitoids
Paleozoic igneous rocks
undivided crystalline rocks
40 N
75°E
30 N
Eurasia
Tibet
India
70 E
80 E
Fig. 1. Geology of the Pamir, emphasizing Cenozoic crystalline basement domes (pink) and Phanerozoic magmatic rocks; compiled from maps by Vlasov et al. (1991), Schwab et al.
(2004), Doebrich and Wahl (2006), Robinson et al. (2007), and our own work from 1996–2011. Inset shows location of Pamir within India–Asia collision zone.
This contribution quantifies the exhumation depths of Pamir highgrade rocks using metamorphic petrology and provides new metamorphic age constraints with zircon, titanite and monazite geochronology.
In conjunction with published work, we find that the high-grade crystalline rocks throughout the central and southern Pamir record
30–40 km of Oligo–Miocene burial and exhumation. In light of recent
studies of xenoliths and seismological imaging, this record suggests
that significant amounts of Pamir crust may have been recycled into
the mantle. This is a tectonic history very different from that of Tibet.
2. High-grade domes of the Pamir
Like Tibet, the Pamir are composed of a sequence of broadly E–W
trending belts (Fig. 1) formed by successive collisions during the Late
Paleozoic–Mesozoic (Schwab et al., 2004). The northern Pamir are a
Paleozoic arc and subduction–accretion complex like the Kunlun
and Hoh Xil–Songpan-Ganzi terranes of northern Tibet, the central
Pamir comprise Paleozoic–Jurassic platform rocks correlative with
the Qiangtang block, and the southern Pamir consist of Proterozoic
gneiss, Paleozoic–Mesozoic metasedimentary rock, and Cretaceous–
Paleogene granitoids equivalent to the Lhasa block (Schwab et al.,
2004; Vlasov et al., 1991) or the Qiangtang block (Burtman, 2010;
Robinson, 2009) in Tibet. Unlike Tibet, these belts have been bent
into an arcuate shape and translated ~300 km northward over the
Tajik Depression (Burtman and Molnar, 1993).
Each of the three major belts of the Pamir includes elongate culminations or domes of medium- to high-grade metamorphic rocks that
collectively make up ~20% of the rock exposed in the Pamir (Fig. 1).
A) Tibet: homogeneous plane strain, no exhumation
51km
initial
thickness
1800 km initial width
70 km
present
thickness
500 km shortening (28%)
1300 km present width
B) Pamir: 600 km shortening of thin crust; no excess crustal material
25-32 km
initial
thickness
70 km
present
thickness
C) Pamir: 600 km shortening of normal crust; 20 km-thick layer of excess crust
40 km
initial
thickness
D) Pamir: 900 km shortening of normal crust; 40 km-thick layer of excess crust
40 km
initial
thickness
70 km
present
thickness
20 km excess
70 km
present
thickness
40 km excess
Fig. 2. Cenozoic exhumation of the Pamir domes from depths of 30–40 km can be explained by inhomogeneous shortening in a narrow zone. A) India–Asia convergence in Tibet can
be explained by homogeneous plane strain. B) 600 km of homogeneous shortening of thin crust does not produce excess crustal material by thickening. C) 600 km of homogeneous
shortening of normal-thickness crust produces 20 km of excess crust by thickening. D) 900 km of homogeneous shortening of normal-thickness crust produces 40 km of excess
crust by thickening. (Crustal configuration before shortening in pale gray and crustal configuration after shortening in dark gray. Pink material removed by tectonic/erosional processes; shown at bottom of crust for convenience, but a component might also be removed from the top.)
413
Table 1
Thermobarometry samples.
Sample
Assemblagea
grt radius (μm)
grt resorption (μm)
N latitude
E longitude
Kurgovat Dome
4715A1
4715B1
4716E1
4717A1
4717A3
4718A1
4718B3
grt bt st ms pl qtz
grtb bt st ms pl qtz
grtb bt st ms pl qtz ilm
grt bt hbl pl qtz ttn
grtb bt st ms pl qtz
grtb bt st ms pl qtz ilm
grt bt st ms pl qtz
217
1065
964
283
2475
1500
1100
7
450
0
3
690
28
0
38°24.597′
38°23.996′
38°26.536′
38°23.299′
38°23.299′
38°19.373′
38°19.021′
71°06.487′
71°08.193′
71°01.380′
71°09.804
71°09.804
71°13.475
71°16.658
Yazgulom Dome
6906A3
grt bt st and pl qtz
550
100
37°58.009′
71°29.784′
Muskol Dome
9919A4b
9919G6
grtb bt ms pl qtz ttn
grt bt ky ms pl qtz
738
1786
0
225
38°21.253′
38°18.922′
74°26.563′
74°26.320′
Shakhdara Dome
4727E1
6821C1
6821M1
6821P2
6822D1
6824G1
6827C2
6831A1
6831C1
6831C2
grt bt sil ms pl qtz
grtb cpx hbl bt pl qtz ttn
grt bt hbl pl qtz ttn
grt cpx hbl bt pl qtz ttn
grt bt ky sil pl qtz
grt bt sil pl qtz rt
grt bt silc pl qtz rt
grt bt silc ms kfs pl qtz
grtb bt silc pl kfs qtz ttn
grtb bt ms ky pl qtz rt
602
600
318
448
986
266
731
500
3850
3716
25
106
50
37
155
22
35
35
0
37
37°29.431′
36°46.516′
36°46.516′
36°46.077′
37°00.299′
37°04.006′
37°11.647′
37°13.466′
37°14.504′
37°14.504′
71°31.816′
71°49.841′
71°49.841′
71°51.238′
72°27.266′
71°32.337′
71°51.842′
72°07.206′
72°11.363′
72°11.363′
a
b
c
Mineral abbreviations after Kretz (1983).
Indicates zoned garnet.
Indicates kyanite interpreted as peak aluminosilicate.
These domes have surface areas of 10's to 100's of square kilometers,
providing substantial windows into processes occurring deep within
the crust during orogenesis and plateau formation. The domes are
dominated by siliciclastic and carbonate metasedimentary sequences,
but also include large volumes of quartzofeldspathic orthogneiss and
granitoids as young as Tertiary (Schwab et al., 2004). The metamorphic mineral assemblages and textures in the domes indicate typical
Barrovian facies-series metamorphism (culminating in kyanite + garnet + biotite with local migmatites) during N–S contraction. This was
followed by syn-tectonic sillimanite or post-tectonic andalusite
growth accompanied by plutonism and N–S stretching (Pashkov
and Dmitriyev, 1981; Peykre et al., 1981; Robinson et al., 2004, 2007
and our observations).
Recent investigations have begun to quantify the exhumation
depths and metamorphic ages of the domes. Robinson et al. (2007)
demonstrated that the 16–8 Ma metamorphic rocks that core the Muztagh Ata dome were exhumed from depths of ~30–35 km to biotite closure to Ar loss by ~8 Ma, and Robinson et al. (2004, 2010) showed that
similar 9 Ma metamorphic rocks that core the Kongur Shan dome were
exhumed to biotite closure at ~1 Ma. On the opposite, western side of
the Pamir, rocks in the western Shakhdara dome were metamorphosed
at ~650 °C and >25 km (Grew et al., 1994) and exhumed to Ar closure
in biotite at 10–9 Ma (Hubbard et al., 1999).
3. Pressure–temperature conditions
We quantified the peak metamorphic conditions in the Kurgovat, Yazgulom, Muskol, and Shakhdara domes (see Fig. 1 and Table 1 for sample
locations and parageneses). Optical microscopy was used first to assess
phase relations and select specific grains for further analysis. X-ray
maps of major-element concentrations in multiple phases in each sample
were then made to assess phase zoning, infer growth and alteration histories, and to determine locations for quantitative compositional profiles.
Mineral compositions were measured by wavelength-dispersive spectrometry on a Cameca SX-50 electron microprobe using a 2 um spot, a
15 kV accelerating voltage and a 15 nA sample current, with natural and
synthetic standards (Appendix Table 1). Mineral formulae and endmember activities were calculated using the AX program of Holland (http://
wserv2.esc.cam.ac.uk/research/research-groups/holland/ax).
Pressures and temperatures were calculated using the Holland and
Powell (1998) thermodynamic dataset and THERMOCALC 3.26, using “mode
1” to calculate intersections of pairs of equilibria and “mode 2” to calculate intersections of multiple reactions (Table 2). Temperatures were derived chiefly from Fe–Mg exchange equilibria: garnet–biotite (GARB,
Ferry and Spear, 1978), garnet–clinopyroxene (GC, Ellis and Green,
1979) and garnet–hornblende (GH, Graham and Powell, 1984), and pressures from the net-transfer reactions plagioclase–garnet–Al2SiO5–quartz
(GASP, Ghent, 1976), biotite–plagioclase–garnet–muscovite (GBMP,
Ghent and Stout, 1981), garnet–hornblende–plagioclase–quartz (GHPQ,
Kohn and Spear, 1990) and garnet–plagioclase–clinopyroxene–quartz
(GADS, Newton and Perkins, 1982). The pressures and temperatures calculated from phase compositions were checked against mineral parageneses in pseudosections calculated with Perple_X v. 7 (Connolly, 1990);
see Appendix text and Appendix Fig. 1.
3.1. Northern Pamir Kurgovat dome
Rocks studied from the Kurgovat dome are dominantly garnet–staurolite–biotite schist with K-white mica (henceforth ‘muscovite’), plagioclase, quartz, and titanite. One mafic rock, 4717A1, contains posttectonic hornblende garben in a matrix of fine-grained plagioclase and
garnet + biotite+ titanite+ quartz (Fig. 3; Table 1). Garnet porphyroblasts are 0.5–5 mm in diameter, subidioblastic, and have strain
shadows. The garnets in the metapelites have core compositions of
alm65–80prp08–11grs06–17sps03–191 and rims of alm77–84prp07–12grs04–12sps01–05; garnets in 4717A1 have alm63prp14grs10sps13 cores and alm63prp12grs10sps16 rims. The garnets are characterized by two types of
major-element zoning (Appendix Fig. 2). 1) Most have a bell-shaped
Mn core–rim profile, with an abrupt increase at the rim; 4716E1 does
1
alm, almandine; grs, grossular; prp, pyrope; sps, spessartine.
414
Table 2
Summary of pressure-temperature determinations.
Sample
THERMOCALC
mode 1
THERMOCALc
mode 2
Perple_X
pseudosections
Methoda
T (°C)
Methoda P (kbar)
Methodb
T (°C)
Methodb P (kbar)
Fit
Corr
Excluded
T (°C)
P (kbar)
Kurgovat
4715A1
GARB
569 ± 61
GBMP
7.6 ± 0.9
582 ± 62
GBMP
7.1 ± 0.9
537 ± 55
GBMP
6.0 ± 0.8
4717A1
4717A3
GH
GARB
648 ± 55
548 ± 57
GHPQ
GBMP
5.8 ± 0.9
6.1 ± 0.8
4718A1
GARB
542 ± 56
GBMP
5.5 ± 0.7
4718B3
GARB
560 ± 59
GBMP
6.2 ± 0.8
0.05
1.42
0.16
0.81
0.35
1.56
0.07
0.59
1.33
0.06
1.36
0.17
1.21
0.85
0.74
0.82
0.69
0.70
0.58
0.57
0.76
0.63
0.81
0.70
0.77
0.62
east
east
east
east
pa
pa
east, parg
–
–
cel, pa
cel, pa
–
–
5–9
GARB
7.6 ± 0.8
8.2 ± 0.9
7.1 ± 0.8
7.6 ± 0.7
6.0 ± 0.8
7.4 ± 1.3
5.8 ± 0.9
6.4 ± 0.8
7.3 ± 0.9
5.5 ± 0.7
6.5 ± 0.9
6.2 ± 0.8
7.2 ± 0.8
575–650
4716E1
gbmpq
gbmpqst
gbmpq
gbmpqst
gbmpq
gbmpqst
ghpq
gbmpq
gbmpqst
gbmpq
gbmpqst
gbmpq
gbmpqst
6.2–8.8
GARB
569 ± 58
594 ± 53
583 ± 59
619 ± 40
541 ± 56
656 ± 82
649 ± 55
575 ± 55
638 ± 55
542 ± 58
626 ± 68
560 ± 57
638 ± 52
600–640
4715B1
gbmpq
gbmpqst
gbmpq
gbmpqst
gbmpq
gbmpqst
ghpq
gbmpq
gbmpqst
gbmpq
gbmpqst
gbmpq
gbmpqst
Yazgulom
6906A3
GARB
597 ± 61
GASP‡
9.7 ± 0.7
gbpqk
588 ± 60
gbpqk
9.4 ± 1.3
0.27
0.88
gbmp
Assembl.
gbpqk
776 ± 83
500–700
770 ± 96
gbmp
gbmpq
gbpqk
10.5 ± 1.2
6.9–9.6(±.07)
11.2 ± 1.8
0.87
1.70
0.56
gbpqs
ghcpq
Assembl.
672 ± 76
554 ± 59
700–750
gbpqs
ghcpq
ghcpq
ghpq
Assembl.
ghcpq
gbpqk
950±170
600–700
678 ± 75
792 ± 85
ghpq
ghpq
ghcpq
gbpqk
6.2 ± 1.0
6.9 ± 0.9
8.7–9.4
(±1.0)
9.7 ± 3.4
3.5–5.3(±2.0)
10.6 ± 1.2
9.5 ± 1.5
gbpqs
gbpqs
720 ± 85
670 ± 62
gbpqs
gbpqs
gbpqk
755 ± 75
gbpqk
Muskol-Sares
9919A4b
9919G6
Assembl.
Shakhdara
4727E1
GARB
6821 C1 GC
6821M1
6821P2
6822D1
6824G1
6827C2
6831A1
6831C1
6831C2
GH
Assembl.
GC
GARB
Assembl.
Assembl.
Assembl.
Assembl.
Assembl.
GARB
Assembl.
650–800
GASP‡
6.6–10.2(±0.6)
703 ± 89
587 ± 56
GASP
GADS
6.9 ± 1.1
7.2 ± 0.9
959±125
600–700
675 ± 65
762 ± 78
700–800
700–800
700–800
700–800
700–800
770 ± 74
700–800
GHPQ
GHPQ
GADS
GASP‡
GASP‡
GASP
GASP‡
GASP‡
GASP‡
GASP‡
GASP‡
11.3 ± 4.2
8.5–9.3(±2.8)
10.6 ± 1.2
9.1 ± 0.6
8.2–9.7(±0.6)
6.3–7.5(±0.8)
8.1–9.6(±0.6)
8.5–10.1(±0.7)
12.6–14.6(±0.4)
9.4±0.5
8.3–9.8(±0.5)
Not definitive
Not definitive
560–680
5–8
570–630
5–6.5
570–655
5–7.8
–
610–650
7–9
0.91
–
0.93
–
–
–
Not definitive
0.50
0.11
1.70
0.66
0.82
–
east
620–730
parg, fact, CaTs
parg, fact, CaTs 680–770
0.32
–
0.15
0.70
0.89
–
0.90
0.90
6.5 ± 1.7
7.7 ± 1.1
0.20
0.02
0.65
0.77
9.1 ± 1.2
0.42
0.85
–
–
parg, fact, CaTs
–
–
–
–
–
–
–
–
620–760
500–680
7–11
5–9
9–13
7–10.5
Not definitive
not definitive
700–740
625–800
720–775
Not definitive
610–720
6.8–8.5
6–10
8–12
8–11
Preferred pressure and temperature in bold face. “Assembl.” indicates qualitative temperature determined from mineral assemblage.
a
Uppercase letters indicate a single equilibrium was used to determine P or T:
GARB, garnet–biotite; GH, garnet–hornblende; GC, garnet–clinopyroxene; GBMP, garnet–biotite–muscovite–plagioclase.
GASP, garnet–aluminum silicate–quartz–plagioclase; GADS, garnet–plagioclase–clinopyroxene–quartz; GHPQ, garnet–hornblende–plagioclase–quartz.
b
Lowercase letters indicate that all equilibria among the named phases were used:
g, garnet; b, biotite; m, muscovite; p, plagioclase; q, quartz; st, staurolite; h, amphibole; c, clinopyroxene; k, kyanite; s, sillimanite.
‡ kyanite was used as peak phase.
“fit”, fit parameter from THERMOCALC.
“corr”, correlation in ± P and ± T from THERMOCALC.
“excluded”, end-member activities excluded from fit (east, eastonite; parg, pargasite; pa, paragonite; cel, celadonite; CaTs, Ca–Tschermak pyroxene; fact, ferroactinolite).
not have a Mn-enriched rim. The Mg# increases outward from the core,
except in the Mn-enriched rims, where it decreases. Ca decreases from
core to rim. 2) Garnets in three samples (4715A1, 4717A1 and 4718B3)
have homogeneous Mn, Mg, and Fe concentrations in their cores. Of
these, samples 4715A1 and 4717A1 display an abrupt increase in Mn
and decrease in Mg# at the rim, whereas 4718B3 is homogeneous
throughout. 4715A1 also has a bell-shaped Ca distribution, whereas
4717A1 is homogeneous in Ca. Plagioclase varies from An18–An40
among the samples; the anorthite component decreases concentrically
from An32 to An29 in 4715A1, whereas all other samples have patchy
zoning. Muscovite is generally 200–400 μm; it is a foliation-forming
mineral in samples 4715B1, 4717A3, and 4718B3, whereas in 4715A1,
4716E1, and 4718A1, it is a minor decussate, post-kinematic phase.
Muscovite has K/K + Na ratios of 70–85, 3.0–3.1 Si atoms per formula
unit (pfu), 0.3–0.5 wt.% TiO2, and Mg#45–65. Staurolite porphyroblasts
are either small (~600 μm), xenoblastic, partially resorbed, and have
undulatory extinction, or are large (~5 mm) idioblastic, and decussate;
they generally display outward zoning from Mg#20 to 16. Biotite in the
metapelites are Mg#47–53, and in the amphibolite they are Mg#56–59.
In most samples (4715A1, 4715B1, 4716E1, 4718B3) biotite decreases
in Mg# with increasing distance from garnet; the remaining samples
exhibit no trend. Amphibole in sample 4717A1 is ferropargasite and
has concentric zoning with 1.2–2.1 wt.% Na2O, 0.1–0.4 wt.% TiO2, and
6.1–6.5 Si atoms pfu.
Major-element zoning was used as an indicator of whether the garnets preserve prograde compositions or were modified by diffusion
and/or retrogression. Bell-shaped Mn profiles were considered characteristic of prograde growth (Hollister, 1966). Mn increases at the garnet rims
were interpreted to reflect resorption (Kohn and Spear, 2000); such
grains also have subidioblastic habits compatible with resorption. Garnets
A: 4717A1
415
B: 4718A1
hornblende
ilmenite
garnet
staurolite
biotite
garnet
muscovite
biotite
500 µm
300 µm
C:6906A3
biotite
D: 9919A4b
garnet
biotite
muscovite
andalusite
garnet
staurolite
1 mm
E: 6822D1
500 µm
F:6821P2
biotite
kyanite
garnet
cpx
biotite
500 µm
sillimanite
500 µm
hornblende
Fig. 3. Photomicrographs of representative Pamir dome samples. (A) Kurgovat garnet-biotite amphibolite with idioblastic garnet; (B) Kurgovat metapelite with idioblastic garnet;
(C) Yazgulom metapelite with late andalusite; (D) Muskol metapelite; (E) Shakhdara metapelite with peak kyanite and late sillimanite in shear bands; (F) Shakhdara clinopyroxenegarnet amphibolite.
with Ca zoning and homogeneous Mg, Fe, and Mn were assumed to have
undergone partial diffusional homogenization (Yardley, 1977).
The method of Kohn and Spear (2000) was used to determine the
magnitude of garnet resorption, to estimate the composition of garnet
rims prior to resorption, and to correct biotite compositions for the net
transfer of Fe and Mg from garnet to biotite. The decrease in Mg# of biotites increasingly distant from garnet was also used as a measure of the effects of garnet resorption on biotite composition. The magnitude of
resorption was calculated to be as much as 30% of the garnet diameter
(Table 1).
To calculate pressures and temperatures (Table 2), we used the
recalculated rim compositions of resorbed garnets with bell-shaped
Mn profiles and the core compositions of homogeneous garnets,
along with the core compositions of staurolite, muscovite, plagioclase,
and biotite distant from garnet. This biotite composition was either
identical to the net-transfer-corrected composition of biotite adjacent
to garnet or gave a lower MSWD (mean square of weighted deviates,
a measure of goodness of fit) in THERMOCALC, leading to the conclusion
that such grains were least affected by garnet resorption and best represent peak conditions. Where available, plagioclase inclusions in homogeneous garnets were used to calculate pressure.
The presence of staurolite + garnet + biotite in the Kurgovat rocks
implies temperatures of ~500–650 °C. Metamorphic temperatures
and pressures calculated using the intersections of GARB and GBMP
(i.e., mode 1) or THERMOCALC mode 2 (excluding staurolite), are
540–650 °C and 5.5–7.6 kbar (Fig. 4). When staurolite is included,
mode 2 results are slightly higher: 600–650 °C at 6.5–8.2 kbar. Calculations for the amphibolite, based on GH and GHPQ, yielded 649 ±
50 °C and 5.8 ± 0.9 kbar. Pseudosections calculated with Perple_X are
compatible with these results.
3.2. Central Pamir Yazgulom dome
One sample from the orthogneiss-dominated Yazgulom dome is a
garnet–andalusite–staurolite hornfels with biotite, plagioclase, and
quartz (Fig. 3; Table 1). The garnet porphyroblasts are subidioblastic,
~1 mm in diameter, and have thin quartzofeldspathic haloes. Idioblastic
staurolite porphyroblasts are ~5 mm long and co-genetic with garnet;
both have muscovite- and biotite-filled strain shadows. Andalusite is
~4 mm long, idioblastic and sector zoned. Biotite is the main foliationforming phase, and ~60 μm long, except for a few ~500 μm grains in
garnet strain shadows and cracks. The garnets have bell-shaped Mn
profiles with an abrupt increase in the outer 40 μm. Mg# increases outward, except in the outer 40 μm, where it decreases; Ca decreases
slightly from core to rim. Garnet cores are alm70prp06grs09sps15, and
rims are alm80prp11grs07sps02. The larger biotite grains have concentric
core–rim zoning from Mg#45 to 43. Staurolite grains are Mg#18.
We followed the same logic detailed for the Kurgovat dome when
calculating pressure–temperature conditions for the Yazgulom sample, using resorption-corrected garnet and biotite rim compositions
16
14
Yazgulom
10
Kongur
Shan
Muskol
50
40
Muztagh
Aata
8
30
Shakhdara
6
4
20
kyanite
sillimanite
andalusite
Kurgovat
pe
li t e
2
0
400
500
depth (km)
pressure (kbar)
12
de mu
hyd sco
rat vit e
ion
me
ltin
g
416
600
700
10
+H
2O
800
0
900
temperature ( C)
Fig. 4. Metamorphic pressures and temperatures indicate that the Pamir domes were
exhumed from lower crustal depths of 30–40 km. Arrows reflect observed and inferred
decompression. Solidi from Vielzeuf and Schmidt (2001).
with plagioclase rims. THERMOCALC mode 2 indicates 588 ± 60 °C and
9.4 ± 1.3 kbar (Fig. 4), indistinguishable from the mode 1 GASP–
GARB intersection. Perple_X returned compatible results. The calculated pressure is well above the stability field of andalusite, suggesting
that the andalusite replaced kyanite.
3.3. Central Pamir Muskol dome
Two samples were analyzed from the eastern part of the Muskol
dome. One (9919 G6) is a garnet–kyanite–biotite schist with muscovite,
plagioclase, and quartz. The garnet is minor, hypidioblastic, and
~3.5 mm in diameter. It has a core of alm79prp13grs05sps02 and a rim
of alm77prp14grs03sps06. It displays a dish-shaped Mg# profile except
in the outer 120 μm, where Mg decreases and Mn increases; Ca decreases outward. Kyanite porphyroblasts are ~1 mm, hypidioblastic
and aligned parallel to the lineation. Plagioclase displays patchy recrystallization in the range An12–19. Biotite is 500–1000 μm long and generally tabular, with compositions of Mg#37–39 that do not vary
systematically with texture.
The second Muskol sample (9919A4b; Fig. 3) is a garnet–biotite
schist with muscovite, plagioclase, quartz, and rare staurolite inclusions
in garnet. The garnet porphyroblasts are 1–2 mm, xenoblastic, and typically elongate; they have dish-shaped Mg# profiles with cores of alm84prp04grs12sps00 and rims of alm73prp10grs18sps00. The more-equant
garnets have a Ca spike in the outer 10% of the grain. Biotite and muscovite are 200–400 μm and aligned parallel to the foliation; large biotite
decrease in Mg#36–34 from core to rim. Muscovite has Mg#48–59,
0.3–1.0 wt% TiO2, 3.1 Si atoms pfu, and K/K + Na of 93–94.
For sample 9919 G6, with homogeneous garnet, mineral cores
were used to determine metamorphic P–T conditions. For sample
9919A4b, garnet has a bell-shaped Mn profile, so mineral rims were
used to calculate the pressure–temperature conditions. In both
cases, the temperature given by GARB is in excess of the probable
mineral-stability conditions. At 500–700 °C, the samples give mode
2 pressures of 6.6–10.2 kbar and 6.9–9.6 kbar, respectively (Fig. 4);
Perple_X returned compatible results for 9919G6.
bands that cut the foliation, except 6831C2, in which kyanite included
in garnet is the only aluminumsilicate. Sample 6822D1 contains kyanite as a matrix phase, in addition to the sillimanite. The metapelitic
garnets have cores of alm50–79prp09–27grs02–10sps02–09 and rims of
alm59–79prp06–28grs02–12sps02–15, whereas the amphibolite garnets
have core and rims of alm51–61prp06–12grs31–35sps02–03 and alm53–63prp05–12grs29–33sps03–05, respectively. Most of the metapelite garnets
have cores that are homogeneous in Mn and Mg, with abrupt increases in Mn and decreases in Mg in the outer b30% of the grain.
Ca decreases from core to rim, except in three samples, in which it
is homogeneous. Two of these samples (6831C1 and 6831C2) exhibit
broad increases in Mg# from core to rim. 6831C2 displays a rim spike
in Mn and drop in Mg# in the outer 5% of the grain. The amphibolite
garnets are homogeneous in Mn and Mg, except for abrupt increases
in Mn and decreases in Mg in their outer 12–25%; garnet in 6821C1
also exhibits a broad increase in Mg out to the rim, where it drops.
Ca decreases broadly rimward.
Plagioclase is a xenoblastic matrix phase in most of the metapelites, but also forms rare inclusions in garnet; most are An11–35 and
have patchy zoning. Two samples however, have plagioclase zoned
outward from An21 to An26, and plagioclase in sample 4727E1 is
zoned rimward from An23 to An15. Rare (b5%) tabular muscovite in
4727E1 is aligned parallel to the foliation, has 3.0 Si atoms pfu,
1.3–1.5 wt.% TiO2, a K/K + Na ratio of 88–90, and Mg#40–44. Biotite
is chiefly a matrix phase, but also occurs in shear bands, in garnet
strain shadows, and as a rare inclusion in garnet. Matrix biotite typically ranges from Mg#41 to 58, except in 4727E1, where it is
Mg#27–30; zoning is typically not systematic.
Clinopyroxene in samples 6821 C1 and 6821P2 is xenoblastic and
exhibits patchy zoning of Mg#63–70, 1.0–3.5 wt.% Al2O3, and
0.3–1.0 wt.% Na2O. Hornblende is ferropargasite and has 6.1–6.3 Si
atoms pfu, 1.1–2.0 wt.% TiO2, and 1.1–1.5 wt.% Na2O. An17–53 plagioclase in the garnet amphibolites is xenoblastic and displays patchy
zoning; 6821 M1 includes minor antiperthite.
For the bulk of the samples that have garnet cores that are homogeneous in Mn and Mg, the core garnet, biotite, and plagioclase compositions were used to calculate temperature and pressure. The
exception is sample 6831C2, which has garnets with bell-shaped
Mn profiles, for which the resorption-corrected garnet rim composition was used, along with the rim compositions of biotite and
plagioclase.
Most of the Shakhdara metapelites give calculated mode 2 temperatures and pressures of 670–800 °C and 6.2–9.9 kbar. Samples 6831A1
and 6831C1 contain so little biotite (b3 vol%), that calculation of their
pre-resorption compositions was deemed unreliable, and temperatures
of 700–800 °C were estimated from their mineral assemblages instead.
For these two samples the GASP reaction gives pressures of
8.5–14.6 kbar. GASP–GARB intersections for the remainder of the metapelites yield temperatures of 700–800 °C and pressures of 6.9–9.7 kbar.
The clinopyroxene-bearing samples yield mode 2 temperatures and
pressures of 554–680 °C and 6.9–10.6 kbar; the GADS–GC intersections
are 587–676 °C and 7.2–10.6 kbar. Mafic sample 6821 M1, which lacks
clinopyroxene, gave mode 2 temperatures of >900 °C—far beyond the
probable stability field of the mineral assemblage; at 600–700 °C the
GHPQ barometer yields pressures of 8.5–9.3 kbar (Fig. 4). Perple_X
returned compatible results for all samples.
3.4. Southwestern Pamir Shakhdara dome
4. Age of metamorphism
Two types of samples were collected from the Shakhdara
dome (Fig. 3; Table 1). Seven are garnet–sillimanite—biotite±kyanite
schist with plagioclase+quartz± muscovite±K-feldspar±titanite±
rutile. Three samples are garnet amphibolite that contain biotite+ plagioclase +quartz + titanite±clinopyroxene; leucocratic layers in outcrop suggest partial melting.
The garnets are hypidioblastic porphyroblasts, 0.4–15 mm in diameter. All the metapelites contain sillimanite along late shear
There is considerable uncertainty in the older literature about the
age of the metamorphic events in the Pamir domes, with suggestions
of Proterozoic to Alpine tectonism hinging on the interpretation of
unclear field relations (Pashkov and Dmitriyev, 1981; Peykre et al.,
1981). Limited radiochronology has documented Mesozoic through
Miocene metamorphism (Hubbard et al., 1999; Robinson et al.,
2004; Robinson et al., 2007; Schwab et al., 2004; Stübner et al.,
2011), but there are relatively few ages from the high-grade domes.
Th-Pb monazite ages indicate Barrovian metamorphism of the northeastern Pamir Kongur Shan and Muztaghata domes between 24 Ma
and 7 Ma, and biotite 40Ar/ 39Ar ages indicate exhumation to upper
crustal levels by 8 Ma at Muztagh Ata (Robinson et al., 2007) and at
1 Ma in the Kongur Shan (Robinson et al., 2010). On the opposite,
western, side of the Pamir, rocks in the Shakhdara dome have biotite
40
Ar/ 39Ar ages of 18–9 Ma (Hubbard et al., 1999).
We performed reconnaissance assessment of the metamorphic
ages of the Kurgovat, Yazgulom, Muskol, and Shakhdara domes
(Fig. 1; Appendix text and Table 2) using U/Th-Pb ages of zircon,
monazite, and titanite; multiple grains, and multiple spots within
some grains, were analyzed in each sample. The analyses were conducted by secondary-ion mass spectrometry (SIMS) at UCLA (following Robinson et al., 2004) and St Petersburg (see Appendix for
analytical details), laser-ablation multi-collector inductively coupled
plasma mass spectrometry (LA-MCICPMS) at the University of Arizona (following Hacker et al., 2006) and UCSB (see Appendix for analytical details). All quoted uncertainties are 2σ and include contributions
from the external reproducibility of the primary reference material.
U-Pb data presentation employed Isoplot (Ludwig, 2003).
4.1. Kurgovat dome
Metamorphic rocks in the Kurgovat dome have been mapped as
Proterozoic with a Carboniferous–Permian cover (Vlasov et al.,
1991). We obtained exclusively concordant, Mesozoic 206Pb/ 238U
and 208Pb/ 232Th ages of 210–195 Ma from monazite in a garnet–two
mica schist (sample 4717A2 in Fig. 5).
4.2. Muskol dome
In four Pamir samples we analyzed titanite with textures that indicate an unambiguous association with metamorphism and deformation. In each case the titanite formed from the breakdown of
ilmenite ± biotite ± plagioclase and underwent polygonization during deformation (Fig. 5). Multiple titanites from one of those samples—a tonalitic gneiss (96Ak2) in the Muskol dome—yielded a U-Pb
age of 16.7 ± 0.3 Ma (Fig. 5).
4.3. Yazgulom dome
Similarly textured titanite from a dioritic gneiss (6906A1) in the
Yazgulom dome gave a U-Pb intercept age of 19.1 ± 0.4 Ma (Fig. 5).
Zircons in a coarse-grained, weakly deformed leucocratic dike
(4727J1) have inherited components as old as 2.5 Ga that are overgrown by euhedral oscillatory zoned portions with Th/U ratios of
0.6–1.0 and 206Pb/ 238U ages as young as 21.2 ± 0.3 Ma. A second,
weakly deformed leucogranite (6818E2) from the northern shear
zone of the Yazgulom dome has monazite that gave 208Pb/ 232Th
ages ranging from ~22 to 19 Ma.
4.4. Shakhdara dome
The giant Shakhdara dome has relict Early Proterozoic basement
and ubiquitous Cretaceous orthogneiss and granitoid (Schwab et al.,
2004). The oldest Cenozoic ages we obtained from the Shakhdara
dome are 30–18 Ma monazite ages from paragneiss (9910A1). Two
other monazite samples from Shakhdara gave ages toward the younger end of that range. A strongly deformed leucocratic gneiss with a
recrystallized metamorphic mineral assemblage (6823A3) has a
mean 208Pb/ 232Th age of 21.1 ± 0.7 Ma, and a paragneiss (6831A3)
from the same locality of petrology sample 6831A1 (and nearby to
6831C) yielded a mean 208Pb/ 232Th age of 22.1 ± 0.3 Ma. A gneissic
pegmatite from the northern Shakhdara dome (4726H1) is almost
identical to dike 4727J1 from the Yazgulom dome: it has inherited
417
zircons as old as 2.5 Ga and ~29 Ma high-Th/U zircons overgrown
by low-Th/U, oscillatory zoned zircon as young as 20.5 Ma. The youngest zircons in a weakly deformed post-metamorphic aplite dike
(6831C3) are 12.0 ± 0.4 Ma. The sample is from the same locality of
petrology samples 6831C1 and 6831C2. Metamorphic titanite from
gabbroic gneiss (6823C1) gave a U-Pb age of 10.1 ± 0.2 Ma.
5. Discussion
5.1. Pressure–temperature conditions
The relatively simple mineralogy and phase zoning of the samples
examined in this study allows reasonably straightforward interpretation of equilibrium compositions and the calculation of P-T conditions
(Fig. 4). The pressure–temperature conditions of the Kurgovat dome
peaked at 600–650 °C and 6.5–8.2 kbar, consonant with the stability
of garnet + staurolite + biotite. The hornfelsic texture and the presence of andalusite are in conflict with the calculated GASP pressure
of 9.4 ± 1.3 kbar for the Yazgulom sample, however, suggesting that
the andalusite overprints an earlier higher pressure metamorphism
recorded in the plagioclase and garnet. Robinson et al. (2004)
reported similar mineral parageneses from the northern hanging
wall of the Kongur Shan dome, and inferred peak metamorphic conditions of 650–700 °C and 4–5 kbar.
Using the data of Robinson et al. (2004, 2007), we calculate pressures and temperatures of 8–9 kbar and 650 °C for Cenozoic metamorphism in the footwall of the Kongur Shan dome, and 9 kbar and 750 °C
for Muztagh Ata. The pressure–temperature estimates for the two
metapelites from the Muskol dome are broadly similar: 6.9–11.7 kbar
and 500–800 °C. Pressure–temperature estimates for the Shakhdara
dome metapelites range from 6.5± 1.7 kbar and 720 ± 85 °C to 13.6 ±
1.0 kbar and 750 ± 50 °C. The highest pressures are consonant with
the presence of kyanite in some samples, but the presence of sillimanite
in shear bands implies continued recrystallization during decompression. Decompression during heating (from ~650 °C and >7 kbar to
650–750 °C and b4 kbar) has been inferred for ultra-magnesian rocks
in the western Shakhdara dome by Grew et al. (1994).
Using a density of 2800 kg/m 3, the maximum exhumation depth
of all the studied Pamir domes is ~30–40 km. At least two domes
show mineralogical evidence of decompression to upper crustal
depths of 14–18 km with little loss of heat, compatible with rapid exhumation and/or exhumation accompanied by plutonism.
5.2. Ages
Our preliminary radiochronology, in conjunction with that of
other workers, documents that the bulk of the deep crust exposed
in the central and southern Pamir was last metamorphosed in the
Oligo–Miocene, although there are local records of earlier events.
The 210–195 Ma monazite ages from the Kurgovat dome are similar
to the 234–192 Ma monazite ages reported for the hanging wall of
the Kongur Shan dome (Robinson et al., 2004); all are part of broader
Triassic–Jurassic orogenic activity that extended across Asia (Roger et
al., 2008; Schwab et al., 2004). In light of the Carboniferous–Permian
depositional age of the Kurgovat dome metasedimentary rocks,
Permo–Triassic burial is required. If peak temperatures in the Kurgovat dome were indeed b700 °C (Fig. 4), it is likely that the monazite
ages record the time of prograde metamorphism (closure to Pb diffusion is >900 °C for 10 μm grains cooled at 10 K/Myr, Cherniak and
Pyle, 2008; Gerdes et al., 2007; McFarlane and Harrison, 2006).
The whole-grain closure temperature of the U-Pb system in titanite is
>650 °C and probably >700 °C (Aleinikoff et al., 2002; Frost et al., 2000;
Pidgeon et al., 1996; Scott and St-Onge, 1995). Our own experience with
>150 multi-titanite U-Pb dates from Norwegian Barrovian facies-series
quartzofeldspathic gneisses (Kylander-Clark et al., 2008; Spencer et al.,
2010) is that the closure temperature is ~650 °C, but that titanite can
j. Schmidt et al. / Earth and Planetary Science Letters 312 (2011) 411-421
418
-r---------------,
0.036
4717A2 Kurgovat monazite (UC5B)
garnet two-mica schist
22
1.0
0.034
6906A I Yazgulom titanite (UC5B)
dioritic gneiss
0.8
::0
~
.0
0-
~
:0 0.032
",0-
0.6
:0
~
~O-
~
0.030
0.4
210-195Ma
0.2
400
0.0 ~=~=:!:::::::::!::::::::::::;:~s:::::::::1
a
100
200
300
400
238 206
0.Q28 .j.o<::............--t---+-~__t-- ......~--l
0.0088 0.0092 0.0096 0.0100 0.0104 0.Q108 0.0112
280 Pbl 232 Th
0.10 .-------~---__,___--__,___,
4727Jl Yazgulom zircon (Arizona)
leucocratic dike
\
concordia age
21.2 ± 0.3 Ma
0.08
7/6-corrected
concordia age:
16.7 ± 0.3 Ma
M5WD= 1.0
6818E2 Yazgulom monazite (UCLA)
weakly deformed leucogranite
25
22-19 Ma
28
0.2
II
0.04
II
17'------------------'
240
400
280
320
360
238 /06 pb
U
23 .---__--:-.....,..,....,...,,....,...---..,....-..,...._,,.,....,..,....--,
6823A3 5hakhdara monazite (UCLA)
leucocratic gneiss
0.006-r---------------:r-,
991 OA I 5hakhdara monazite (UC5B)
paragneiss
3
+
O.Oos
24
~
~ 23
~
'"
mean = 21.1 ± 0.7 Ma
M5WD= 3.5
~
-'=
19
:0
~
-'=
t--
N
M
N
:0
i
~
30-18 Ma
0.003
17
0
22
15
0.002.j....~-+-~-+-__t_-+_-_+_~
..........J
mean = 22.1 ± 0.3 Ma
MSWD = 0.35
i
'" 21
i
6831A3 Shakhdara monazite (UCLA)
paragneiss
21 ' - - - - - - - - - - - - - - - - - '
0.0007 0.0009 0.0011 0.0013 O.OOlS 0.0017 0.0019
280 Pbl 232 Th
0.12
0.20
.---4-7--2-6":'"H--l~5--h-a":'"k--h-:d-ar-a-z":'"ir-c-o-n-:(A-:--:riz-o-n-a":'")'
6831 C3 Shakhdara zircon (St. Petersburg)
post-metamorphic aplite
concordia age
0.16
12.0 ± 0.4 Ma
0.18
gneissic pegmatite
0.10
0.08
f.
'"
~
28.8 ± 0.3 Ma
concordia age
Th/U = 0.3
20.5 ± 0.3 Ma
. - - - - Th/U = <0.05
0.16
0.12
f.
:0
~a.. 0.08
32
0.04
0.06
0.04
0.02
~
fA
5
20
180
__
~'-~_~
220
260
238 /06 pb
U
~~~_...J
300
340
0.00
7/6-corrected
concordia age:
10.1 ±0.2 Ma
MSWD= 1.3
:0
~O-
10
~
o
0.12
N
~
0.08
0.02
0.00 L-_ _
;g
0.10
N
0.06
:0
,...0~
0.14
~
;g
6823Cl Shakhdara titanite (UCSB)
gabbroic gneiss
0.20
200
400
238U;206 pb
15
600
14
13
460
SOO
S40
S80
238 206
pb
U/
12
11
10
0.04~~:t::~~=~:::!:=~=~t::t
420
620
660
Fig. 5. Cenozoic ages from the Yazgulom, Muskol and 5hakhdara domes and aJurassic age from the Kurgovat dome. Zircon and titanite data shown on Tera-Wasserburg concordia: the
titanite data are not corrected for common Pb. Monazite LA-MCICPM5 data displayed as Th-Pb/U-Pb concordia, and monazite SIMS data shown as 208Pb/232Th ages. Uncertainties are 2a.
Numbers on concordia are ages in Ma. Note that because of the emphasis on the Cenozoic importance of these rocks, some older data are not plotted. Foliation-forming polycrystalline
titanite grains in 6906AI are shown in upper right.
effectively remain closed for at least 10 Myr at 800°C; this is in accord
with experiments (Cherniak, 1993) suggesting a closure temperature
> 780°C at the Myr timescale for 500 f-U1l crystals. If so, the titanite ages
from the Muskol and Yazgulom domes likely indicate prograde metamorphism and deformation (and thus, probably burial) at 16,7 and 19.1 Ma,
respectively. Monazite and zircon in the Yazgulom dome indicate latestage leucogranite crystallization at 22-19 Ma (Fig,S),
The range of monazite, zircon, and titanite ages from the giant
Shakhdara dome indicates that Cenozoic magmatism and metamorphism were underway by 30-29 Ma, and monazite ages of -22 Ma
from three separate locations imply continuing prograde metamorphism; zircon ages from a post-metamorphic aplite suggest that metamorphism terminated locally by 12 Ma. Deformed titanite with an age
of 10 Ma may reflect local prograde recrystallization, but it is similar
j. Schmidt et al. / Earth and Planetary Science Letters 312 (2011) 411-421
enough to biotite 4oAr/39Ar ages of 10-9 Ma (Hubbard et aI., 1999) it
may instead reflect cooling-related closure.
Similar ages for Cenozoic metamorphism in the Muztagh Ata dome
are indicated by 33-22 Ma ages from low-Th/U zircons, 30-8 Ma monazite matrix grains (mostly 15-8 Ma), and 8 Ma 4oAr;J9Ar ages for biotite (Robinson et aI., 2007). It is reasonable to associate the -10 Ma
transition in the Shakhdara and Muztagh Ata domes from high-grade
metamorphism (zircon, monazite, titanite) to cooling (biotite) with exhumation, and this corresponds to the beginning of syn-orogenic coarse
clastic sedimentation in the foreland basins, which are uppermost Miocene to Pliocene (Tajik depression and Pamir frontal range, Leonov,
1977; Schwab et aI., 1980; and Tarim Basin,lin et aI., 2003). Exhumation
in the Kongur Shan dome to biotite closure is even younger, - 1 Ma
(Robinson et aI., 2010).
419
As noted in the Introduction, the disparity in N-S dimensions of
the Pamir and Tibet requires the Pamir to have undergone considerably more Cenozoic strain in response to India-Asia convergence:
-19-28% shortening in Tibet and 55-64% shortening in the Pamir
(Fig. 2). (The oroclinal shape of the Pamir precludes major E-W
shortening, so these plane-strain calculations are minima.) The Cenozoic deep exhumation of the Pamir crystalline domes documented
here can be interpreted in the light of two endmember models that
differ only in their pre-collisional crustal thickness (Fig. 6).
5.3.1. Model A
If the crust of the Pamir prior to the India-Asia collision was only
25-32 km thick - as inferred by Burtman and Molnar (1993) from a review of stratigraphic sections and seismic data from the Tajik Depression - the inferred 55-64% shortening of the Pamir could have been
accommodated solely by homogeneous plane-strain vertical thickening
(Fig. 6A). Each ofthe deeply exhumed Cenozoic Pamir domes must then
represent a zone of unusually great exhumation, and each must be compensated by a corresponding zone of less exhumation.
5.3. Convergence and crustal recycling
Three aspects suggest that Pamir continental crust may still be
being recycled into the mantle: intermediate-depth seismicity, crustal xenoliths, and the exhumation depths of the domes. The Pamir are
underlain by intermediate-depth seismicity indicative of active subduction from the north and south. The south-dipping zone extends
beneath the northern Pamir to a depth of 150 km and has been interpreted to be the result of intracontinental subduction, based on the
absence of exposed ocean crust in the region (Burtman and Molnar,
1993; Hamburger et aI., 1992). A north-dipping zone west of the
Pamir beneath the Hindu Kush has been interpreted to mark
-700 km of subducted Indian lithosphere (Negredo et aI., 2007),
and slow velocities to at least 150 km depth are compatible with
the subduction of continental crust (Roecker, 1982). In the Miocene,
crustal xenoliths of Asian affinity were erupted from mantle depths
of 90 km in the southeastern Pamir (Gordon et aI., in review; Ducea
et aI., 2003; Hacker et aI., 2005). Regardless of whether these xenoliths are the result of lower crustal foundering or subduction erosion
(Hacker et aI., 2011), they indicate definitively that Pamir crust wasand may still be-locally present at depths of 90 km, well below the
present Moho.
5.3.2. Model B
Alternatively, the pre-Cenozoic Pamir crust might have been substantially thicker-e.g., 35-40 km as inferred for the southern Tian
Shan (Bagdassarov et aI., 2011), or, perhaps even locally 70 km
thick as speculated for southern and central Tibet (Kapp et aI., 2005;
Murphy et aI., 1997). The inferred 55-64% shortening would then indeed require the removal of vast amounts of crust-equivalent to
20-40 km thickness (Fig. 2C-D) from the Pamir orogen by i) subhorizontal extrusion along strike, ii) erosion, or iii) return to the mantle
(Fig. 6B). In this model, each of the Cenozoic Pamir domes was derived from the base of the pre-collisional normal-thickness crust or
from middle or deeper levels of the crust thickened during the
collision.
Substantial orogen-parallel extrusion in the Pamir is precluded by
the prevalence of N-S stretching lineations at all exposed structural
levels. Erosion certainly contributed to the removal of crust from
the Pamir system, but is unlikely to have been the principal mechanism because i) low-grade sedimentary and volcanic sections are
Model A: 600 km shortening of thin crust; no excess crustal material
1) 32 km thick pre-collisional crust LI
2) contraction; burial offuture domes >30 km
..J
L-
3) deeper contraction, shallower extension; exhumation of dome
----=:~_-::;::::;:::::--=-----------'
'------.........-:-E:::-~
-----------
~
-
-----ro~
Model B: 900 km shortening of normal crust; 40 km thick layer of 'excess' crust
1) 40 km thick crust
IL2) contraction; burial of future domes >40 km
_
'--------~~~
..¥'" -"",--:;;:=--
3) deeper contraction, shallower extension; exhumation of dome
I
---~
Fig. 6. The implications of the 30-40 km exhumation depths of the Cenozoic Pamir domes depend on the magnitude of shortening and the thickness of the pre-collisional crust.
A) From a pre-collisional crustal thickness of 32 km, India-Asia convergence of 600 km can build a 70 km-thick crust with plane strain and no out-of-plane extrusion. erosion,
or recycling of crust. Each dome (dark pink) represents a zone of unusually great exhumation and must be compensated by a corresponding zone of less exhumation. B) From a
pre-collisional crustal thickness of 40 km. India-Asia convergence of 900 km builds a 70 km thick crust plus another 40 km of 'excess' crust that must be removed by out-ofplane extrusion, erosion. or recycling. Each dome was derived from the base of the pre-collisional normal-thickness crust or from middle or deeper levels of the crust thickened
during the collision.
420
exposed across the Pamir (Vlasov et al., 1991); ii) zircon (U/Th)-He
ages from the northern Pamir are Late Cretaceous to Eocene
(Amidon and Hynek, 2010); and iii) the entire Tajik Depression contains only ~ 500,000 km 3 of Cenozoic sediment (Brookfield and
Hashmat, 2001), whereas just the northern half of the Pamir has a
crustal volume ten times larger (400 km × 250 km × 70 km). Recycling of Pamir crust into the mantle is therefore likely to have occurred and is probably ongoing (Fig. 6B). This is documented by i)
the late Miocene eruption of crustal xenoliths from 90–100 km
depth in the southeastern Pamir (Hacker et al., 2005), and ii) earthquake hypocenters and seismic wavespeeds (Roecker, 1982).
6. Conclusions
Crystalline high-grade domes are exposed over a wide area of the
Pamir. Thermobarometry yields peak P–T conditions for the domes of
6–14 kbar and 500–800 °C, corresponding to exhumation depths of
30–40 km. New titanite, zircon and monazite U/Th-Pb data plus existing geochronology indicate that most of the metamorphism—and,
therefore, also the exhumation—occurred in the Oligo–Miocene.
Both of these findings indicate that the Pamir are quite different
than the Tibetan plateau, which has undergone minimal Cenozoic exhumation. The extensive and widespread exhumation of the deep
Pamir crust is almost certainly a result of the India–Asia convergence
being absorbed over a relatively narrow north–south distance.
Whether the exhumation of the domes was accompanied by the subhorizontal extrusion, erosion, or foundering of significant crustal
mass hinges on the pre-collisional crustal thickness and the magnitude of shortening within the Pamir.
Acknowledgments
This material is based on work in the TIPAGE (Tien Shan–Pamir Geodynamic) initiative supported by the Deutsche Forschungsgemeinschaft
(bundle 443 and Ra442/34) and the US National Science Foundation
(EAR-0838269). The ion microprobe facility at UCLA is partly supported
by a grant from the Instrumentation and Facilities Program, Division of
Earth Sciences, National Science Foundation. Sample collection and interpretation of field relationships was completed during joint field work
with M. Gadoev, R. Gloaguen, S. Gordon, J. Hofmann, E. Kanaev, I. Oimahmadov, D. Rutte, and K. Stanek. Alex Robinson and an anonymous reviewer contributed to the clarity and completeness of the presentation;
Alex also generously shared a manuscript in review that helped crystallize our understanding of the northeastern Pamir.
Appendix A. Supplementary data
Supplementary data to this article can be found online at doi:10.
1016/j.epsl.2011.10.034.
References
Aleinikoff, J.N., Wintsch, R.P., Fanning, C.M., Dorais, M.J., 2002. U-Pb geochronology of
zircon and polygenetic titanite from the Glastonbury Complex, Connecticut, USA;
an integrated SEM, EMPA, TIMS, and SHRIMP study. Chem. Geol. 188 (1–2),
125–147.
Amidon, W.H., Hynek, S.A., 2010. Exhumational history of the north central Pamir. Tectonics 29, TC5017. doi:10.1029/2009TC002589.
Bagdassarov, N., Batalev, V., Egorova, V., 2011. State of lithosphere beneath Tien Shan
from petrology and electrical conductivity of xenoliths. J. Geophys. Res. 116.
doi:10.1029/2009JB007125.
Brookfield, M.E., Hashmat, A., 2001. The geology and petroleum potential of the North
Afghan Platform and adjacent areas (northern Afghanistan, with parts of southern
Turkmenistan, Uzbekistan and Tajikistan). Earth Sci. Rev. 55 (1–2), 41–71.
Burtman, V.S., 2010. Tien Shan, Pamir, and Tibet: history and geodynamics of Phanerozoic oceanic basins. Geotectonics 44, 388–404.
Burtman, V.S., Molnar, P., 1993. Geological and geophysical evidence for deep subduction of continental crust beneath the Pamir. Geol. Soc. Am. Spec. Pap. 281, 1–76.
Cherniak, D.J., 1993. Lead diffusion in titanite and preliminary results on the effects of
radiation damage on Pb transport. Chem. Geol. 110 (1–3), 177–194.
Cherniak, D.J., Pyle, J.M., 2008. Th diffusion in monazite. Chem. Geol. 256 (1–2), 52–61.
Connolly, J.A.D., 1990. Multivariable phase diagrams an algorithm based on generalized
thermodynamics. Am. J. Sci. 290, 666–718.
DeCelles, P.G., Robinson, D.M., Zandt, G., 2002. Implications of shortening in the Himalayan fold–thrust belt for uplift of the Tibetan Plateau. Tectonics 21. doi:10.1029/
2001TC001322.
DiPietro, J.A., Pogue, K.R., 2004. Tectonostratigraphic subdivisions of the Himalaya; a
view from the west. Tectonics 23 (5), 20.
Doebrich, J.L., Wahl, R.R., 2006. Geologic and Mineral Resource Map of Afghanistan. US
Geol. Surv.
Ducea, M.N., Lutkov, V., Minaev, V.T., Hacker, B., Ratschbacher, L., Luffi, P., Schwab, M.,
Gehrels, G.E., McWilliams, M., Vervoort, J., Metcalf, J., 2003. Building the Pamirs:
the view from the underside. Geology 31, 849–852.
Ellis, D.J., Green, D.H., 1979. An experimental study of the effect of Ca upon garnetclinopyroxene Fe-Mg exchange equilibria. Contrib. Mineral. Petrol. 71, 13–22.
Ferry, J.M., Spear, F.S., 1978. Experimental calibration of the partitioning of Fe and Mg
between biotite and garnet. Contrib. Mineral. Petrol. 66, 113–117.
Frost, B.R., Chamberlain, K.R., Schumacher, J.C., 2000. Sphene (titanite): phase relations
and role as a geochronometer. Chem. Geol. 172, 131–148.
Gerdes, A., Liu, F., Weyer, S., Brey, G., Anonymous, 2007. Chronological history of UHP
rocks from the Chinese Continental Scientific Drilling; a multi-methodical approach. Geochim. Cosmochim. Acta 71 (15S), A317.
Ghent, E.D., 1976. Plagioclase–garnet–Al2SiO5–quartz: a potential geobarometer–
geothermometer. Am. Mineral. 61, 710–714.
Ghent, E.D., Stout, M.Z., 1981. Geobarometry and geothermometry of plagioclase–biotite–garnet–muscovite assemblages. Contrib. Mineral. Petrol. 76, 92–97.
Gordon, S.M., Luffi, P.I., Hacker, B.R., Valley, J.W., Spicuzza, M., Kozdon, R., Kelemen, P.,
Ratschbacher, L., Minaev, V., in review. The thermal structure of continental crust
in active orogens: Insight from Miocene eclogite- and granulite-facies xenoliths
of the Pamir. J. Metamorph. Geol.
Graham, C.M., Powell, R., 1984. A garnet-hornblende geothermometer; calibration,
testing, and application to the Pelona Schist, Southern California. J. Metamorph.
Geol. 2, 13–31.
Grew, E.S., Pertsev, N.N., Yates, M.G., Christy, A.G., Marquez, N., Chernosky, J.C., 1994.
Sapphirine + forsterite and sapphirine + humite-group minerals in an ultramagnesian lens from Kuhi-lal, SW Pamirs, Tajikistan: are these assemblages forbidden? J. Petrol. 35, 1275–1293.
Hacker, B.R., Luffi, P., Lutkov, V., Minaev, V., Ratschbacher, L., Plank, T., Ducea, M.,
Patiño-Douce, A., McWilliams, M., Metcalf, J., 2005. Near-ultrahigh pressure processing of continental crust: Miocene crustal xenoliths from the Pamir. J. Petrol.
46, 1661–1687.
Hacker, B.R., Wallis, S.R., Ratschbacher, L., Grove, M., Gehrels, G., 2006. High-temperature geochronology constraints on the tectonic history and architecture of the
ultrahigh-pressure Dabie-Sulu orogen. Tectonics 25, TC5006. doi:10.1029/
2001JB001129/2005TC001937.
Hacker, B.R., Kelemen, P.B., Behn, M.D., 2011. Differentiation of the continental crust by
relamination. Earth Planet. Sci. Lett. 307, 501–516.
Hamburger, M.W., Sarewitz, D.R., Pavlis, T.L., Popandopopulo, G.A., 1992. Structural and
seismic evidence for intracontinental subduction in the Peter the First Range, central Asia. Geol. Soc. Am. Bull. 104, 397–408.
Holland, T.J.B., Powell, R., 1998. An internally consistent thermodynamic data set for
phases of petrological interest. J. Metamorph. Geol. 16, 309–343.
Hollister, L.S., 1966. Garnet zoning: an interpretation based on the Rayleigh fractionation model. Science 154, 1647–1651.
Hubbard, M.S., Grew, E.S., Hodges, K.V., Yates, M.G., Pertsev, N.N., 1999. Neogene cooling and exhumation of upper-amphibolite-facies ‘whiteschists’ in the southwest
Pamir Mountains, Tajikistan. Tectonophysics 305, 325–337.
Jin, X., Wang, J., Chen, B., Ren, L., 2003. Cenozoic depositional sequences in the piedmont of the west Kunlun and their paleogeographic and tectonic implications. J.
Asian Earth Sci. 21, 755–765.
Johnson, M.R.W., 2002. Shortening budgets and the role of continental subduction during the India–Asia collision. Earth Sci. Rev. 59, 101–123.
Kapp, P., Yin, A., Harrison, T.M., Ding, L., 2005. Cretaceous–Tertiary shortening, basin
development, and volcanism in central Tibet. Geol. Soc. Am. Bull. 117, 865–878.
Kohn, M.J., Spear, F.S., 1990. Two new geobarometers for garnet amphibolites, with applications to southeastern Vermont. Am. Mineral. 75, 89–96.
Kohn, M.J., Spear, F., 2000. Retrograde net transfer reaction insurance for pressure–
temperature estimates. Geology 28, 1127–1130.
Kretz, R., 1983. Symbols for rock-forming minerals. Am. Mineral. 68, 277–279.
Kylander-Clark, A.R.C., Hacker, B.R., Mattinson, J.M., 2008. Slow exhumation of UHP terranes: titanite and rutile ages of the Western Gneiss Region, Norway. Earth Planet.
Sci. Lett. 272, 531–540.
Le Pichon, X., Fournier, M., Jolivet, L., 1992. Kinematics, topography, shortening, and extrusion in the India–Eurasia collision. Tectonics 11, 1085–1098.
Leonov, J.G., 1977. Materialien zur Geologie der Afgano-Tadschikischen Depression.
Akademy of Sciences of U.S.S.R., problem commission 9, working group 3–3: “the
tectonic regime of molasse-forming periods” report, 50 pages (in Russian).
Ludwig, K.R., 2003. Isoplot 3.00. A Geochronological Toolkit for Microsoft Excel: Berkeley Geochronology Center Special Publication, 4.
McFarlane, C.R.M., Harrison, T.M., 2006. Pb-diffusion in monazite: constraints from a
high-T contact aureole setting. Earth Planet. Sci. Lett. 250, 376–384.
Mechie, J., Kind, R., Saul, J., 2011. The seismological structure of the Tibetan Plateau
crust and mantle down to 700 km depth. Geol. Soc. Spec. Publ. 353, 109–125.
Mechie, J., X. Yuan, X., Schurr, B., Schneider, F., Sippl, C., Ratschbacher, L., Minaev, V.,
Gadoev, M., Oimahmadov, I., Abdybachaev, U., Moldobekov, B., Orunbaev, S., Negmatullaev, S., in review. Crustal structure along a profile across the Pamir and
southern Tien Shan as derived from project TIPAGE wide-angle seismic data. Geophys. J. Int.
Murphy, M.A., Yin, A., Garrison, T.M., Dürr, S.B., Chen, Z., 1997. Significant crustal shortening in south-central Tibet prior to the Indo–Asian collision. Geology 25, 719–722.
Negredo, A., Replumaz, A., Villaseñor, A., Guillot, S., 2007. Modeling the evolution of
continental subduction processes in the Pamir–Hindu Kush region. Earth Planet.
Sci. Lett. 259, 212–225.
Newton, R.C., Perkins, D.I., 1982. Thermodynamic calibration of geobarometers based
on the assemblages garnet–plagioclase–orthopyroxene (clinopyroxene) –quartz.
Am. Mineral. 67, 203–222.
Pashkov, B.R., Dmitriyev, 1981. Muskol crystalline massif (central Pamir). Int. Geol. Rev.
24, 285–296.
Peykre, Y.B., Pokhvisneva, Y.A., Zotov, I.A., 1981. The Muskol metamorphic complex of
the Central Pamir. Int. Geol. Rev. 24, 297–303.
Pidgeon, R.T., Bosch, D., Brugier, O., 1996. Inherited zircon and apatite U-Pb systems in
an Archean syenite from southwestern Australia: implications for U-Pb stability of
titanite. Earth Planet. Sci. Lett. 141, 187–198.
Ratschbacher, L., Merle, O., Davy, P., Cobbold, P., 1991. Lateral extrusion in the eastern Alps,
part 1: boundary conditions and experiments scaled for gravity. Tectonics 10, 245–256.
Robinson, A.C., 2009. Geologic offsets across the northern Karakorum fault: implications for its role and terrane correlations in the western Himalayan-Tibetan orogen. Earth Planet. Sci. Lett. 279, 123–130.
Robinson, A.C., Yin, A., Manning, C.E., Harrison, T.M., Zhang, S., Wang, X., 2004. Tectonic evolution of the northeastern Pamir; constraints from the northern portion of the Cenozoic
Kongur Shan extensional system, western China. Geol. Soc. Am. Bull. 116, 953–973.
Robinson, A.C., Yin, A., Manning, C.E., Harrison, T.M., Zhang, S.-H., Wang, X.-F., 2007. Cenozoic evolution of the eastern Pamir: implications for strain-accommodation
mechanisms at the western end of the Himalayan-Tibetan orogen. Geol. Soc. Am.
Bull. 119, 882–896.
Robinson, A.C., Yin, A., Lovera, O., 2010. The role of footwall deformation and denudation in controlling cooling age patterns of detachment systems: an application to
the Kongur Shan extensional system in the Eastern Pamir, China. Tectonophysics
496, 28–43.
421
Roecker, S.W., 1982. Velocity structure of the Pamir–Hindu Kush region: possible evidence of subducted crust. J. Geophys. Res. 87, 945–959.
Roger, F., Jolivet, M., Malavieille, J., 2008. Tectonic evolution of the Triassic fold belts of
Tibet. C.R. Geosci. 340, 180–189.
Schurr, B., Yuan, X., Mechie, J., Sippl, C., Schneider, F., Minaev, V., Abdybachaev, U.,
2009. A temporary seismological network across the Pamir and Tien Shan mountain ranges. EOS Trans. Am. Geophys. Union T43C–T2102C.
Schwab, G., Katzung, G., Ludwig, A.O., Lützner, H., 1980. Neogene MolasseSedimentation in der Tadschikischen Depression (tadschikische SSR). Z. Angew.
Geol. 26/5, 225–238.
Schwab, M., Ratschbacher, L., Siebel, W., McWilliams, M., Lutkov, V., Minaev, V., Chen,
F., Stanek, K., Nelson, B., Frisch, W., Wooden, J.L., 2004. Assembly of the Pamirs:
age and origin of magmatic belts from the southern Tien Shan to the southern Pamirs and their relation to Tibet. Tectonics 23, TC4002. doi:10.1029/2003TC001583.
Scott, D.J., St-Onge, M.R., 1995. Constraints on Pb closure temperature in titanite based
on rocks from the Ungava Orogen, Canada; implications for U-Pb geochronology
and P-T-t path determinations. Geology (Boulder) 23 (12), 1123–1126.
Spencer, K., Hacker, B.R., Kylander-Clark, A.R.C., Cottle, J.M., Ginsburg, A., 2010. Determining the exhumation history of a giant UHP terrane using titanite geochronology, western gneiss region, Norway. Geol. Soc. Am. Abstr. Programs 42.
Stübner, K., Ratschbacher, L., Weise, C., Pfänder, J., Gloaguen, R., Hacker, B., Dunkl, I.,
Jonckheere, R., Stanek, K., Minaev, V., 2011. Miocene gneiss domes and synorogenic extension in the Pamir. Abstract EGU General Assembly, Vienna, Austria.
Vielzeuf, D., Schmidt, M.W., 2001. Melting relations in hydrous systems revisited: application to metapelites, metagraywackes and metabasalts. Contrib. Mineral. Petrol. 141, 251–267.
Vlasov, N.G., Y.A. Dyakov, and E.S. Cherev, Geological map of the Tajik SSR and adjacent
territories, 1:500,000, Leningrad, 1991.
Yardley, B.W.D., 1977. An empirical study of diffusion in garnet. Am. Mineral. 62,
793–800.

Cenozoic deep crust in the Pamir - Earth Science

Transcription

Similar documents